Tailoring liquid water permeability of diffusion layers in fuel cell stacks

ABSTRACT

A fuel cell stack ( 31 ) includes a plurality of fuel cells ( 9 ) each having an electrolyte such as a PEM ( 10 ), anode and cathode catalyst layers ( 13, 14 ), anode and cathode gas diffusion layers ( 16, 17 ), and water transport plates ( 21, 28 ) adjacent the gas diffusion layers. The cathode diffusion layer of cells near the cathode end ( 36 ) of the stack have a high water permeability, such as greater than 3×10 −4  g/(Pa s m) at about 80° C. and about 1 atmosphere, whereas the cathode gas diffusion layer in cells near the anode end ( 35 ) have water vapor permeance greater than 3×10 −4  g/(Pa s m) at about 80° C. and about 1 atmosphere. In one embodiment, the anode gas diffusion layer of cells near the anode end ( 35 ) of the stack have a higher liquid water permeability than the anode gas diffusion layer in cells near the cathode end; a second embodiment reverses that relationship.

TECHNICAL FIELD

The liquid water permeability of the anode and cathode gas diffusionlayers are tailored for each cell according to its position within thefuel cell stack, so as to promote movement of water toward watertransport plates and away from catalysts, especially cathode catalysts,taking into account that water moves toward the cooler part of the stackduring the cooling (and possibly freezing) process. By controlling thewater movement of each cell during the cooling process, the cold startperformance of the stack can be improved.

BACKGROUND ART

It has been previously suggested that the startup procedure for a fuelcell stack at subfreezing temperature is hampered by the presence of icein the porous catalyst layers of the electrodes. The ice prevents thereactant gases from reaching certain parts or even all of theelectrodes' catalyst layer surfaces. To avoid such a situation, manyproposals have been made for removing all of the water and water vaporfrom the stack when the stack is being shut down so that there is nopossibility of ice being present upon re-establishing operation. Suchsystems are expensive, awkward, and quite time-consuming, and arecertainly not at this time well suited for fuel cell power plants usedin vehicles. The dry out of the cell stack assembly which is necessaryfor good cold start performance, can result in severe membrane stress,leading to untimely membrane failure.

Other approaches to the catalyst/ice problem include all sorts ofheating methodologies, which are also expensive, cumbersome and requiretoo much time, and are not well suited for vehicular applications.

SUMMARY

Recognition of the fact that water in a fuel cell stack will tend tomigrate toward the freezing front (toward the lower temperature along atemperature gradient), the liquid water permeability (water permeance)of gas diffusion layers (GDLs) is made lower than normal where acatalyst layer will be at a lower temperature than its correspondingwater transport plate (WTP), and greater than normal where a catalystlayer will be at a higher temperature than its corresponding watertransport plate. This gradation in GDL water permeance tailors thecapability of the fuel cells to conduct water away from catalyst layerstoward water transport plates, at either end of the stack, thusminimizing startup problems due to ice blockage of gas transport to thecells' catalyst layers.

Herein, the “anode end of the stack” and “anode end” are defined as theend of the stack at which the anode of the fuel cell closest to that endis closer to that end than the cathode of the closest fuel cell.

Specifically, at the anode end of the stack, each cells' anode watertransport plate is closer to the stack end plate and therefore each WTPwill be cooler than its associated anode catalyst layer, as the stackcools upon shutdown. As a result, during a shutdown procedure, waterinventory normally tends to migrate through the anode gas diffusionlayer (GDL) toward the water transport plate. Since this water migrationis beneficial to fuel cell restart capability from a frozen condition,the GDL adjacent to each anode catalyst layer, at the anode end of thestack, has a greater than normal liquid permeability in order to promotewater migration away from the anode catalyst layer.

On the other hand, at the anode end of the stack, the cathode catalystlayer is closer to the anode end plate and therefore colder than itsassociated cathode water transport plate. As a result, during a shutdownprocedure, the fuel cell water inventory will normally migrate from thewater transport plate (where it is abundant) toward the cathode catalystlayer. In order to impede this water flow, the cathode GDL is providedwith lower than normal water liquid permeability.

When the stack temperature is below freezing, at the anode end of thestack, and freezing occurs in the small pores of the anode WTP, adecrease in the liquid pressure occurs drawing water out of the anodecatalyst layer (toward the anode water transport plate) so that theanode catalyst layer dries out. On the other hand, as the water freezesin the small pores of the cathode catalyst layer, water is drawn out ofthe cathode water transport plate, through the cathode GDL and into thecathode catalyst layer. As the water is drawn into the cathode catalystlayer, the ice pressure increases, forcing small hydrophobic pores ofthe cathode catalyst layer, which are normally empty, to fill with ice.Once the pores of the cathode catalyst layer are filled, they are verydifficult to empty. This cathode condition results in the performanceloss seen after a boot strap start from freezing temperatures. Whilethis phenomenon also works to fill the anode catalyst layer at thecathode end of the stack, the fuel cell is more tolerant of anodecatalyst layer flooding due to rapid hydrogen/oxygen kinetics andhydrogen diffusion capability. Also, anode catalyst layer flooding ismore easily recovered during normal fuel cell operation due toelectro-osmotic drag of water from the anode electrode toward thecathode.

This water movement problem also exists in fuel cell power plants notutilizing water transport plates since there are small pores in thecatalyst layers and water can move within the membrane electrodeassembly itself. However, there is much less water inventory availableto move within the cell (there is some liquid water in the GDLs and inthe gas channels), so the problem is less severe.

The opposite situation occurs at the other end of the stack.

At the cathode end of the stack, the anode catalyst layer is closer tothe cathode stack end plate and therefore cooler than its associatedanode water transport plate as the stack cools upon shutdown. As aresult, during a shutdown procedure, the fuel cell water inventorymigrates from the water transport plate toward the anode catalyst layer.In order to impede this flow, the anode GDL at the cathode end of thestack is provided with lower than normal water permeability.

At the cathode end of the stack, the cathode water transport plate iscloser to the cathode stack end plate, and therefore there is migrationof water from the cathode catalyst towards the cathode water transportplate. To enhance this flow, the cathode GDL at the cathode end of thestack is provided with higher than normal permeability.

The arrangement herein may be utilized in several cells at each end ofthe stack, or up to one-half of the stack at each end of the stack ifdesired, but generally need not be utilized in every cell in the stack.For instance, applying the principles herein to 8 or 10 cells at eitherend of a stack will typically be sufficient to avoid ice blockage ofreactant gases in the end cells. The arrangement may be used in fuelcell stacks with solid polymer electrolytes or liquid electrolytes. Thearrangement may be used in power plants with external, internal, or somecombination of water management systems, including evaporative cooling.

A second embodiment achieves a significant reduction in performanceproblems related to flooding electrode catalyst layers by takingadvantage of the tolerance to flooding at the cell anodes referred tohereinbefore. In the second embodiment, the GDLs of cathodes and anodesat the anode end of the stack have lower than normal water permeability,while the GDLs of the cathodes and anodes at the cathode end of thestack have higher than normal water permeability.

A third embodiment also achieves a significant reduction in performanceproblems related to flooding of electrode catalyst layers by takingadvantage of the tolerance to flooding at the cell anodes referred tohereinbefore. In the third embodiment, the GDLs of cathodes and anodesat the anode end of the stack have low water permeability, while at thecathode end of the stack, the GDLs of the cathodes have high waterpermeability and the GDLs of the anodes have low water permeability.

Other variations will become apparent in the light of the followingdetailed description of exemplary embodiments, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fractional, side elevation view of a pair of contiguous fuelcells of one exemplary form with which the present arrangement may beutilized.

FIG. 2 is a stylized, graphical depiction of a fuel cell stack and theGDL water permeability relationships in a first embodiment of thepresent arrangement relating to anodes and cathodes, at the anode endand at the cathode end of the stack.

FIG. 3 is a stylized, graphical depiction of a fuel cell stack and theGDL water permeability relationships in a second embodiment of thepresent arrangement relating to anodes and cathodes, at the anode endand at the cathode end of the stack.

FIG. 4 is a stylized, graphical depiction of a fuel cell stack and theGDL water permeability relationships in a third embodiment of thepresent arrangement relating to anodes and cathodes, at the anode endand at the cathode end of the stack.

MODE(S) OF IMPLEMENTATION

Referring to FIG. 1, a pair of fuel cells of one form with which thepresent arrangement may advantageously be utilized each include a protonexchange membrane 10 (PEM). On one surface of the PEM 10 there is ananode catalyst layer 13 and on the opposite surface of the PEM there isa cathode catalyst layer 14. Adjacent the anode catalyst layer there isa porous anode gas diffusion layer 16 (GDL), and adjacent the cathodecatalyst layer there is a porous cathode GDL 17. Fuel is supplied to theanode in fuel reactant gas flow field channels 20 within an anode watertransport plate 21 (WTP), which is sometimes referred to as a fuelreactant flow field plate. The water transport plate 21 is porous and atleast somewhat hydrophilic to provide liquid communication between waterchannels, such as channels 24 (which may be formed in the oppositesurface of the water transport plate from the fuel channels 20) and fuelchannels 20.

Similarly, air is provided through oxidant reactant gas flow fieldchannels 27 which are depicted herein as being orthogonal to the fuelchannels 20. The air channels 27 are formed on one surface of thecathode water transport plates 28 which have characteristics similar tothose of water transport plates 21.

The catalysts are conventional PEM-supported noble metal coatingstypically mixed with a perfluorinated polymer, such as that sold underthe tradename NAFION® which may or may not also contain teflon. The PEM10 consists of a proton conductive material, typically perfluorinatedpolymer, such as that sold under the tradename NAFION®. Water istransferred from the water channels 24 through the porous, hydrophilicWTPs 21 and the anode GDL 16, to moisturize the PEM. At the catalystlayer, a reaction takes place in which two hydrogen diatomic moleculesare converted catalytically to four positive hydrogen ions (protons) andfour electrons. The protons migrate through the PEM to the cathodecatalyst. The electrons flow through the fuel cell stack out of theelectrical connections and through an external load, doing useful work.The electrons arriving at the cathode combine with two oxygen atoms andthe four hydrogen ions to form two molecules of water. The reaction atthe anode requires the infusion of water to the anode catalyst, whilethe reaction at the cathode requires the removal of product water whichresults from the electrochemical process as well as water draggedthrough the PEM from the anode by moving protons (and osmosis).

The cathode catalyst layer 14 is similarly porous and the GDL 17 isporous to permit air from the channels 27 to reach the cathode catalystand to allow product and proton drag water to migrate to the cathodeWTP, where the water will eventually reach the water channels 24. In apower plant having an external water management system, the water willexit the stack for possible cooling, storage and return to the stack asneeded.

Referring to FIG. 2, a fuel cell stack 31 is depicted at the top with aplurality of contiguous fuel cells 9 pressed together between end plates32. There is an anode stack end 35 and a cathode stack end 36. The fuelcells typically operate at temperatures above 60° C. (140° F.) inenvironments which are typically 37° C. (100° F.) or lower. In somecases, the environment may be below the freezing temperature of water.Whenever the fuel cell is shut down, the ends of the fuel cell cool downmore quickly than the center of the fuel cell, particularly where thestack is surrounded either by external reactant gas manifolds orinsulation. Thus, each cell that is not at the end of the stack issomewhat warmer than an adjacent cell which is closer to the end of thestack. Thus, there is an increasing temperature gradient from the endsof the stack toward the center of the stack, with the stack becomingwarmer towards the center cells. This temperature gradient also existsbetween the different parts of each fuel cell near the ends of thestack, as indicated in FIG. 2. Along the lower part of FIG. 2, the lightdashed arrows indicate water migrating as a function of temperaturegradient, and the darker dashed arrow indicates migration resulting fromice, as described hereinbefore.

Along the bottom of FIGS. 2-4, the various GDLs are identified asdesirably having higher than normal liquid water permeability or lowliquid water permeability, according to the foregoing descriptions.

Variations in liquid water permeability may be achieved by adjusting thecharacteristics of the paper of which the GDL is formed, which istypically a mixture of fiber and particulate carbon, such as one of thereadily available TORAY® papers, having suitable porosity and pore sizefor proper passage of reactant gas. The degree of hydrophobicity is thenadjusted by adding an appropriate thin coating of a suitable polymer,such as PTFE. On the other hand, the paper can be produced with adesired hydrophobicity by including a suitable thermoplastic resin inthe paper making process.

In the embodiment of FIG. 3, the water permeability of the anode GDLs atboth ends of the stack supports water migration toward the anodecatalysts, relying on the ability of anodes to clear water away and torecover performance. However, the water permeability of the cathode GDLsat both ends of the stack resists water migration toward the cathodecatalysts.

The embodiment of FIG. 4 takes advantage of the tolerance to flooding atthe cell anodes. In FIG. 4, the GDLs of cathodes and anodes at the anodeend of the stack have low water permeability, while at the cathode endof the stack, the GDLs of the cathodes have high water permeability andthe GDLs of the anodes have low water permeability.

As used herein, the gas diffusion layer is defined as being one or morelayers interposed between an electrode and a water transport plate. Itis sometimes called a support layer. Sometimes a support layer isreferred to as having a substrate which is adjacent to the watertransport plate as well as a microporous layer that is adjacent to thecatalyst. Typically, the substrate will be relatively hydrophilicwhereas the adjacent microporous layer will be relatively hydrophobic.Thus, a support comprising a substrate and a microporous layer will bereferred to herein as a gas diffusion layer (GDL). On the other hand, agas diffusion layer may only comprise what is essentially the same as asubstrate layer of a two-layer gas diffusion layer. In this arrangement,the gas diffusion layer can be a single layer or it can be a dual layeror even have more than two layers.

The thickness, or porosity or wettability of the support layer may beadjusted in any combination to provide a greater or lesser impediment tothe migration of water. However, the control of water permeability mayalso be imparted by the characteristics, particularly pore size andhydrophobicity, of the microporous diffusion layer, rather than thesupport.

The adjustments between high liquid water permeability GDLs and lowliquid water permeability GDLs may, in some cases, be made on a relativebasis, that is to say, having the anode end, cathode GDLs and thecathode end, anode GDLs with a water permeability which is somepercentage of the water permeability of the anode end, anode GDL and thecathode end, cathode GDL. But generally, the absolute liquid waterpermeability of each GDL (or groups of GDLs) will be selected withoutregard to the liquid water permeability of other GDLs of the stacksubject to other, different operational characteristics. Low liquidwater permeability may range from near zero up to about 3×10⁻⁴ g/(Pa sm) and high liquid water permeability may exceed normal, which is about3×10⁻⁴ g/(Pa s m).

Herein, the anode water transport plate 21 is illustrated as beingseparated from the cathode water transport plate 28, meeting at a seamwhich together form water passageways 24. However, it is possible thatthe water transport plates 21, 28 may be combined in some fashionwithout altering the advantage of the present arrangement.

1. Apparatus comprising: a fuel cell stack (31) including a plurality ofcontiguous fuel cells (9) compressed between a pair of end plates (32),each of said fuel cells comprising an electrolyte (10) with an anodecatalyst layer (13) on one surface of the electrolyte and a cathodecatalyst layer (14) on a second surface of the electrolyte, an anode gasdiffusion layer (16) adjacent the anode catalyst and a cathode gasdiffusion layer (17) adjacent the cathode catalyst, an anode watertransport plate (21) adjacent the anode gas diffusion layer and acathode water transport plate (28) adjacent the cathode gas diffusionlayer; said stack having an anode end (35) and a cathode end (36);characterized by: the cathode gas diffusion layer of cells near thecathode end having higher water permeability than the cathode gasdiffusion layer of cells near the anode end.
 2. Apparatus according toclaim 1 further characterized in that: the cathode gas diffusion layer(17) of cells near the cathode end (36) have water permeability greaterthan about 3×10⁻⁴ g/(Pa s m) at about 80° C. and about 1 atmosphere. 3.Apparatus according to claim 1 further characterized in that: the waterpermeability of the cathode gas diffusion layer (17) of cells near theanode end (35) is lower than 3×10⁻⁴ g/(Pa s m) at about 80° C. and about1 atmosphere.
 4. Apparatus comprising: a fuel cell stack (31) includinga plurality of contiguous fuel cells (9) compressed between a pair ofend plates (32), each of said fuel cells comprising an electrolyte (10)with an anode catalyst layer (13) on one surface of the electrolyte anda cathode catalyst layer (14) on a second surface of the electrolyte, ananode gas diffusion layer (16) adjacent the anode catalyst and a cathodegas diffusion layer (17) adjacent the cathode catalyst, an anode watertransport plate (21) adjacent the anode gas diffusion layer and acathode water transport plate (28) adjacent the cathode gas diffusionlayer; said stack having an anode end (35) and a cathode end (36);characterized by: the anode and cathode gas diffusion layers (16, 17) ofcells near the anode end (35) having water permeability which is lowerthan the water permeability of the anode and cathode gas diffusionlayers of cells near the cathode end (36).
 5. Apparatus according toclaim 4 further characterized in that: the anode gas diffusion layer(16) of cells near the anode end (35) having water permeability greaterthan about 3×10⁻⁴ g/(Pa s m) at about 80° C. and about 1 atmosphere. 6.Apparatus according to claim 4 further characterized in that: the waterpermeability of the anode gas diffusion layer (16) of cells near thecathode end (36) is less than about 3×10⁻⁴ g/(Pa s m) at about 80° C.and about 1 atmosphere.
 7. Apparatus according to claim 1 furthercharacterized by: the anode gas diffusion layer (16) of cells near theanode end (35) having water permeability which is less than the waterpermeability of the anode gas diffusion layer (16) of cells near thecathode end (36).
 8. Apparatus according to claim 1 furthercharacterized by: the anode gas diffusion layer (16) of cells near theanode end (35) having water permeability which is equal to the waterpermeability of the anode gas diffusion layer (16) of cells near thecathode end (36).
 9. Apparatus according to claim 8 furthercharacterized in that: the water permeability of the anode gas diffusionlayer (16) of cells near the cathode end (36) and the anode gasdiffusion layer (16) of cells near the anode end (35) is greater thanabout 3×10⁻⁴ g/(Pa s m) at about 80° C. and about 1 atmosphere. 10.Apparatus comprising: a fuel cell stack (31) including a plurality ofcontiguous fuel cells (9) compressed between a pair of end plates (32),each of said fuel cells comprising an electrolyte (10) with an anodecatalyst layer (13) on one surface of the electrolyte and a cathodecatalyst layer (14) on a second surface of the electrolyte, an anode gasdiffusion layer (16) adjacent the anode catalyst and a cathode gasdiffusion layer (17) adjacent the cathode catalyst, an anode watertransport plate (21) adjacent the anode gas diffusion layer and acathode water transport plate (28) adjacent the cathode gas diffusionlayer; said stack having an anode end (35) and a cathode end (36);characterized by: the anode gas diffusion layer (16) of cells near theanode end (35) having water permeability which is less than the waterpermeability of the anode gas diffusion layer (16) of cells near thecathode end (36).
 11. Apparatus according to claim 9 furthercharacterized in that: the anode gas diffusion layer (16) of cells nearthe anode end (35) have liquid water permeability less than about 3×10⁻⁴g/(Pa s m) at about 80° C. and about 1 atmosphere.
 12. Apparatusaccording to claim 9 further characterized in that: the water vaporpermeability of the anode gas diffusion layer (16) of cells near thecathode end (36) is greater than about 3×10⁻⁴ g/(Pa s m) at about 80° C.and about 1 atmosphere.
 13. Apparatus according to claim 10 furthercharacterized by: the cathode gas diffusion layer of cells near thecathode end having higher water permeability than the cathode gasdiffusion layer of cells near the anode end.